Aircraft air-to-ground and ground-to-air voice radio communications take place within the VHF band. Airliners are equipped with dual VHF transmitter/receivers ("transceivers") allowing a choice of any two of 720 channels spaced at 25 kHz in the range 118 to 136 MHz, and ground facilities communicate with aircraft on any single channel in that range. It is, therefore, quite normal to have as many as eight aircraft with their transceivers operating on a single VHF channel, i.e. that of a ground facility such as air traffic control.
A problem which arises from such a situation is that if two transmitters operate at the same time on the same channel, any receivers tuned to that channel will receive either a jumble of words or a high pitched whistle. The operators of the two transmitters will be unaware that their transmissions are interfering one with the other. While this typically causes no more than a minor nuisance, with a request for each message to be repeated separately, it can cause messages to be misunderstood, with possibly disastrous consequences.
To overcome this type of problem, it has been proposed to include blocking means arranged to prevent transmission of a signal by the apparatus while a signal of predetermined strength is being received by the apparatus. Apparatus of this type is disclosed in U.K. Patent Specification No. 1 480 418 in the name of Budapesti Radiotechnikai Gyar. However, in an emergency, or where it is apparent that another transmitter has been left operating in error, some means of over-riding the operation of the blocking means is required. In Budapesti U.K. Patent Specification No. 1 480 418, reference is made to switching the apparatus to an operational mode in which the blocking means is made ineffective. It will be apparent that such a provision might well render the provision of a blocking control circuit in radio transceivers ineffective as a way of overcoming the original problem of preventing interfering sumultaneous transmissions, since it is possible that many transceivers would be left permanently switched to the operational mode in which the blocking means is ineffective, either by oversight or even deliberately. This is particularly true in the case of aircraft radio apparatus, where the inconvenience or even potential danger in having to operate a control on the transceiver set would dissuade aircrew from repeating the operation to cancel the over-ride when not required.